The present invention generally relates to a reinforced composite structure. More particularly, the invention is directed to a composite structure which is reinforced and assembled using composite thread. The composite thread, by holding together the formed details, allow assembly of the composite structure while in a staged condition such that the entire structure can be co-cured (curing of each member of the structure at the same time) and bonded in one operation. In addition, by strategically sewing the stitches of composite thread in the structure, peel forces on the structural joint are minimized.
Usage of composite materials in structures has been increasing rapidly. This is particularly true in the aerospace industry where the lightweight, high-strength characteristics of these materials are particularly desirable. In this art, strong, lightweight, tough, self-sustaining sheet material has been developed, these materials being composed as a class of a resinous sheet reinforced with layers of continuous, lineally aligned, parallel filaments. These sheets may be formed as a single layer sheet or as multi-layer laminates, and thereafter thermoset to tough, hard, exceptionally strong panels, or skins for aircraft and the like.
As initially formed, these sheet materials are flexible and deformable, providing panel-forming members which can be draped or otherwise conformed to various shapes and thereafter cured, by thermosetting, upon the application of heat and pressure thereto, to tough, strong skins or panels of permanent shape retention and having exceptional tensile strength imparted by the continuous filament reinforcing.
Panels or skins made up of these materials are exceptionally strong, lightweight structural units. However, structures made by the inclusion of reinforcing ribs or the like on such panels have heretofore seen limited applicability. Presently, these skins or panels are typically joined by first curing the sheet material into the desired shape and adhesively bonding a web or rib structure to the panel through the medium of a thermosetting adhesive.
However, as previously stated, uncured composite structure is flexible and deformable and when curing large combined structural members, such as an aircraft wing box, difficulties have been encountered in holding the parts in place, especially when transporting the structure to an autoclave or oven. Further, is has been found that tension loads, at unacceptably low values, on the cured panel or skin of the composite structure result in "peel" forces which tend to separate the web and panel of the structure at their interface (joint).